Photographic plate



May 4, 1965 H. A. FRoMsoN 3,181,461

PHOTOGRAPHIC PLATE Filed May 23, 1963 INVENTOR. Ahn/4P@ A'. 690445 0^/ ATTORNEY S United States Patent O 3,181,46l PHOTGGRAPHC PLATE Howard A. Fromson, Rogues Ridge Road, Weston, Conn. Filed May 23, 1963, Ser. No. 282,810 16 (Ilaims. (Si. lOl-149.2)

The present invention relates to a photographic plate for use in planographic printing and the method of making the same.

Photographic printing plates are known, made by applying to an aluminum surface of an aluminum sheet an aqueous solution of an alkali metal silicate, such as sodium silicate to form a Water insoluble layer, which r.,- sults from the reaction of said silicate with the aluminum, and which presents a hydrophilic, organophobic (ink repelling) surface, and applying to said layer a coating of light-sensitive water-soluble diazo resin. Upon exposure of the plate to light through a stencil or negative, this diazo resin will react in the exposed portion to form an image of water-insoluble, hydrophobic and organophilic material bonded to the hydrophilic silicate layer. After exposure, the parts of the diazo resin coating, which have not been exposed to light, and which are still Watersoluble, are completely Washed away, leaving the parts of the hydrophilic, organophobic, silicate layer in the non-image areas exposed. In the printing press, upon application of Water and then printing ink to the plate constructed as described, the exposed hydrophilic, organophobic, silicate layer will adsorb water and reject ink and the image-forming diazo resin coating will be receptive to the ink and will reject the water. The plate can thereby be employed for planographic or lithographie printing in the press.

Aluminum surfaces do not have comparatively high resistance to abrasion or corrosion. Moreover, when using sheets with aluminum surfaces, light-sensitive materials other than diazo resins, such as polyvinyl alcohol, containing potassium, sodium, cupric or ammonium bichromate, although highly desirable for photographic printing, will have comparatively little adherence to the silicate-aluminum reaction layer and will easily wash or rub oli the printing plate, either during the developing stage, or after a comparatively short period of use in the printing press.

One object of the present invention is to provide a photographic printing plate which is superior to those known in the art.

Another object of the present invention is to provide a new and improved photographic printing plate, which is highly resistant to abrasion and corrosion, which is inert to most solvent systems and acids, such as those that might be used in conjunction with the production or development of the printing plate or With the operation of printing, which has a high degree of selectivity in its adsorption and repellency of Water and ink, so that it will reproduce pictures with extreme clea'rness, brilliancy and fidelity.

As an important feature of the present invention, in the production of the printing plate, instead of employing a sheet free from aluminum oxide coatings, there is employed an aluminum base sheet having an aluminum oxide coating thereon, produced, for example, by anodizing the aluminum sheet to form a porous anodic coating thereon consisting of aluminum oxide. An aqueous solution of alkali metal silicate and preferably an aqueous solution of sodium silicate, is then applied to this porous aluminum oxide coating, which adsorbs the silicate. Desirably the application of the silicate solution to the oxide surface is carried out under conditions to seal at the same time the aluminum oxide coating. It is preferred to apply the silicate solution before or during sealing; if

samer Patented May 4,- 1965 desired, sealing of the aluminum oxide coating can be dispensed with entirely.

It is believed that the alkali metal silicate and the aluminum oxide chemical react to form an aluminosilicate layer. This layer is Water-insoluble and is intimately integrated into the aluminum oxide layer or if the silicate solution is applied at sealing temperature, it is intimately integrated into the aluminum oxide hydrate layer produced by such sealing. Although the sealing of the aluminum oxide surface has the elect of reducing the porosity of the surface of the metal sheet, the aluminosilicate layer bonded thereto constitutes a somewhat porous structure believed to be in the nature of a commercial zeolite, and has the unique properties of a molecular sieve with an affinity for molecules of a deiinite size and shape. For that reason, this aluminosilicate layer has high selectivity for water and since inks constitute large molecules, these large molecules are too large to enter into the pores of the layer, making said layer highly hydrophilic and extremely organophobic. This selectivity is exercised by the reception of the water into the aluminosilicate layer while rejecting the organo or oleo particles (ink) because of their sizes, and by the repellent action of the water adsorbed in this layer upon these particles. Because of this double-barrel action of the aluminosilicate layer, the maximum rejection of the organo particles is exercised with a minimum of water, and the consequent use of less Water reproduces pictures of greater brilliancy.

Also, the layer formed by the reaction of aluminum oxide and alkali metal silicate has the desirable property of adsorbing nitrogenous compounds, such as nitrogen and ammonia, released by certain types of light-sensitive substances when exposed to light.

On the layer formed by the reaction of the alkali metal silicate and the aluminum oxide is applied light-sensitive material, which is soluble in a solvent before being exposed to light and which upon exposure to light becomes insoluble in said solvent as well as in water. The lightsensitive material, for example, may be a water-soluble, light-sensitive diazo resin, or a polyvinyl hlm-forming resin. It is preferred, however, to employ a polyvinyl alcohol containing a light-sensitizing agent, which causes the polyvinyl iilm to become insolubilized upon exposure to light; ammonium bichromate is the preferred agent, although potassium, sodium or cupric bichromate can also be used. Polyvinyl alcohol as the basis for the light-sensitive material is highly desirable because of its high chemical and dimensional stability and because it is inexpensive in comparison to such light-sensitive materials as diazo resins. A mixture of polyvinyl alcohol and polyvinyl acetate may also be employed as a basis for the light-sensitive coating, and even polyvinyl acetate alone may be used. However, these are not as `eiective and desirable as the polyvinyl alcohol alone with the insolubilizing agent.

It has been found that although the sensitized imageforming diazo resin will adhere to a layer formed by the reaction of aluminum and an alkali metal silicate, a lightsensitive material such as polyvinyl alcohol containing a bichromate, after exposure to light, will wash or rub olic easily. On the other hand, the latter light-sensitive material, applied to a layer consisting of the reaction product of alkali metal silicate and aluminum oxide, after exposure to light, will adhere tirmly and indefinitely to the plate.

In the process or" making the photographic printing plate of the present invention, an oxide coating is formedA on the ysurface of an aluminum base sheet in any man? ner well known in the art. By aluminum base sheet" is meant a sheet, which aside from its oxide coating contains essentially aluminum in uncornbined form and which may contain alloying substances. The aluminum sheet may have a thickness of, for example, between .005 inch dienaar and .G inch and theV aluminum oxide surface formed thereon should preferably be less than .0005 inch thick to prevent crazes or lissures in the printing plate when applied to the printing roller of a prin-ting press.

The aluminum oxide surface may be formed by anodizing and for that purpose, the aluminum sheet may be anodized at 70 to 100 F. by immersion in an electrolyte comprising 15% by weight of sulfuric acid in water solution for a period of one to fifteen minutes, while applying a voltage of approximately l2 to 18 volts.

The alkali metal silicate applied to the anodized surface of the aluminum sheet maybe sodium silicate, potassium silicate or calcium silicate, but is preferably Vsodium or potassium silicate and between the latter-two, sodium silicate is preferred, especially because it is less expensive.

It is preferred to 4carry out the application of the silicate to the anodized surface of the aluminum sheet at a temperature which makes it readily soluble and reactive and at that temperature, the anodized surface is :sealed in the Ypresence of water. 'For that purpose, the anodized aluminum sheet is dipped in an aqueous sodium silicate solution containing from '1/2 to 10% by weight of sodium silicate at a .temperature of Abetween 155 and 212 F. for 1 to ltlminutes.

The aluminum sheet treated as described is then rinsed and dried.

The light-sensitive material applied over the aluminosilicate layer is desirably an aqueous solution of polyvinyl alcohol highly hydrolyzed to cause it to go easily into solution. The concentration of polyvinyl alcohol in solution is desirably from 8 to 15% by weight. Added to this polyvinyl alcohol solution is ammonium bichromate desirably in an amount ofV 1/2 to 11/2% by weight of the entire mixture of polyvinyl alcohol, water and ammonium bichromate. In place of the ammonium bichromate, equivalent amounts of the sodium, potassium or cupric bichromate may be used. The mixture of polyvinyl alcohol solution andbichromate Yhas the consistency of a light gum and can be applied in any suitable manner at substantially room temperature, as for example, by` a roll coating method or byvwiping with a soaked sponge or cheesecloth. The resulting coating over the layer of aluminum oxide-sodium silicate reaction product is dried, preferably by a stream of warm air at a temperature between 80-100" F.

To develop the sensitized plate formed ais described, the plate with the insoluble layer of aluminum oxidesodium silicate reaction product on the anodized surface of the aluminum sheet, and the coating of light-sensitive material over said layer, is exposed to ultra-violet light through a stencil or negative for a short period or time suicient to insolubilize the light-exposed portions of said coating, as for example, fora period of l to 5 minutes, depending on the intensity of the ultra-violet light. The light exposed portions of this coating will react to form an image of water-insoluble, hydrophobic and organophilic material lirmly bonded to the layer of alu- .minum oxide-sodium silicate reaction product having permanent hydrophilicy, while the portions of the coating not exposed to the ultra-violet light remain unreacted and water-soluble. These unreacted and water-soluble portions of the coating are removed, as for example, by-

Washing with lukewarm water, to define the non-image areas of the printing plate and to expose in these areas, the water-insoluble, hydrophilic, organophobic layer formed by the reaction of the aluminum oxide and the' silicate.

The polyvinyl alcohol resin containing the bichromate is desirable as the light-sensitive coating material because it is sensitive and selective in its reception ofink and its repellency of Water when insolubilized, it forms a strong tenacious lilm on the plate stronglyadhering to the aluminum oxide-sodium silicate reaction layer, it has great chemical and dimensional stability, and as already incertain aspects of the invention are concerned, a light- CII sensitive water-soluble Vdiazo resin may be employed. Such a Water-soluble diazo resin, when reacted by the light, expels nitrogen in the process of becoming insolubilized and the reaction layer of aluminum oxide-sodium silicate is useful in connection with such a resin because it adsorbs the expelled nitrogenous material readily and thereby aids in the insolubiliziing reaction.

The water-soluble diazo resin which may be employed in connection with the production of the printing plate of the present invention is easily available commercially ormay be manufactured as described in U.S. Patent No. 2,714,066 and its concentration and manner of application in the process or" making the printing plate may be as indicated in said patent. After the plate with the diazo resin coating is exposed to ultra-violet light, as for example, from l to 5 minutes, through a stencil or negative, it may be developed by wiping with a solution of gum arabic to dissolve and remove the diazo resin not light-reacted and not insolubilized. .Y Whether polyvinyl alcohol-ammonium bichromate mixture or diazo Vresin is employed as the material for the light-sensitized coating, the printing plate is characterized by maximum water receptivity and ink repellency in the non-image areas, and because of the fact that aV :sheet having an aluminum base and an aluminum oxide coating thereon has been employed in the manufacture of the printing plate, the printing surface has extreme hardness, having a Mobs hardness of about 7, is highly resistant to scratching or abrasion, and has non-scumming characteristics. Because of the tine grain on the printing plate due to the aluminum oxide coating on the sheet employed in the making tot' the sheet, there is added ineness and fidelity in the reproductions printed by the plate. Y

Also, as far as certain aspects of the invention are concerned, ,a polyvinyl alcohol-polyvinyl acetate mixture may be employed, or any other suitable light-sensitive material may be used, as long as it Vis applied to a layer formed by the reaction of an alkali metal silicate with an aluminum oxide surface.

The following specific examples are described in connection with the accompanying drawings, in which FIG. l shows in section and on an enlarged scale an aluminum base sheet which is employed in the manufacture of the printing V'plate embodying the present invention and which consists of an aluminum b-ase having van aluminum oxide coating thereon;

FIG. 2 shows in section the aluminum base sheet of FIG. l after a solution of sodium silicate has been applied to the aluminum oxide coating on the sheet to form an insoluble layer resulting from the reaction of the aluminum oxide 'and sodium silicate;

FIG. 3 shows in section the aluminum base sheet of FIG. 2 after a soluble coating of light-sensitive material has been applied thereto, and

FIG. 4 shows a section of the aluminum base sheet of FIG. 2 after the soluble light-sensitive coating has been light-reacted and rendered insoluble in the image areas and after the soluble non-image areas of the coating have been washed away.

Example 1 An `aluminum sheet having a thickness of about 0.005 inch is degreased by a grease solvent which may consist essentially of trichloroethylene or a mild alkaline cleaner, such as a solution of sodium hydroxide consisting of 4 oz. of hydroxide per gallon of water. The degreased sheet after rinsing with water is then anodized at F. in an electrolyte comprising 15% H2804 `at a voltage of 15 volts fora period of about 8 minutes, to produce a sheet 10 having analuminum base 11 and a coating 12 of aluminum oxide as shownrin FIG. 1. The anodized sheet is then sealed in an aqueous solution of sodium silicate (NazO-SiOZ) with -a silica to alkali ratio of 3.22 to 1 and a concentration of 5% at a temperature of 200 F. for a period of 5 minutes, and this produces an insoluble, hydrophilic, organophobic layer 13 on the sheet, as shown in FIG. 2, resulting from the reaction of the sodium silicate and the aluminum oxide coating. After this treatment, the plate is thoroughly washed and dried.

The silicate treated sheet is next treated with a solution containing 2O parts by Weight of polyvinyl alcohol 8S to 90% hydrolyzed (DuPont Elvanol 52-22), 128 parts by Weight of Water, and 8%. parts by Weight of an 18% aqueous solution of ammonium bichromate. This mixture is applied with a sponge in the form of a thin coating 14, as shown in FIG. 3. The plate so treated is then dried by a stream of Warm air at about 90 F. and constitutes a sensitized lithographie plate.

Exposure of the sensitized lithographie or planographic plate may be carried out under a source of ultra-violet light through a negative or stencil for 2 minutes. After such exposure, the plate is Washed with lukewarm Water to remove the soluble non-image areas of the coating While retaining the insoluble image areas 15, as shown in FIG. 4. The plate so formed and developed can then be used in a printing press Where it is subjected alternately to Water and printing ink.

Example 2 An aluminum base sheet with an aluminum oxide coating and a silicate layer as described in Example 1, is treated with a mixture containing ll parts by weight of a polyvinyl acetate water emulsion (DuPont Elvacet 8l- 900), l part by weight of polyvinyl alcohol in solid powder form (DuPont Elvanol 52-22), 4 parts by weight of Water, and 7 parts by weight of 18% aqueous solution of 'ammonium bichromate. The treatment thereafter is as described in Example l.

By adding polyvinyl acetate to the light-sensitive mixture, the mixture is easy to apply, as for example, by a squeegee, and produces a coating which is free from pinholes and which gives tine image detail.

Example 3 An aluminum base sheet with an aluminum oxide coating and a silicate layer as described in Example l, is treated with a mixture containing 100 parts by Weight of polyvinyl acetate emulsion Containing 55% solid and the rest water (DuPont Elvacet 81-900) and 50 parts by Weight of 18% aqueous solution of ammonium bichromate. The treatment thereafter is as described in Example l, except that a commercial form of solvent developer is employed to remove the non-image areas of the polyvinyl coating from the plate, namely Insta-Plate Developer (Sinclair and Valentine).

Example 4 An aluminum base sheet with an aluminum oxide coating and a silicate layer as described in Example 1J is treated with a mixture containing 2 parts by Weight of a polyvinyl acetate solution consisting of 60% polyvinyl acetate in methanol (DuPont Elvacet 6G-05), 6 parts by weight of ethyl alcohol for thinning since the polyvinyl solution is very thick, and one part by weight of 18% ammonium bichromate. The treatment thereafter is as described in Example 3.

Example 5 To the polyvinyl acetate mixture of Examples 3 and 4, there is added 1 part by Weight of dibutyl phthalate to plasticize the polyvinyl acetate. The dibutyl phthalate is added to the polyvinyl acetate mixture by stirring for 3G minutes. The process is otherwise as described in Examples 3 'and 4.

p d Example 6 The process of Example 1 is followed, except that for the light-sensitive coating material, the diazo resin is ernployed. This diazo resin is of the type and is applied in the manner described in the aforesaid Patent 2,714,066 and the plate sensitized in this manner can be developed in the manner described in said patent.

While the invention has been described with particular reference to specic embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

l. A sensitized photographic plate comprising an aluminum sheet which has lbeen treated to produce an aluminum oxide coating thereon, a water-insoluble, hydrophilic, organophobic layer on said sheet resulting from the reaciton of the aluminum oxide coating and an alkali metal silicate applied to said coating, and a light-sensitive coating over said layer having one solubility in relation to a solvent in a state before exposure to light and another solubility in relation to said solvent in another state after exposure to light, said light-sensitive material being soluble in said solvent in one of said states and being insoluble in said solvent and in water, hydrophobic and organophilic in its other state.

2. A sensitized photographic printing plate comprising an aluminum base sheet having a surface which has been treated to form an aluminum oxide coating on said surface, a water-insoluble, hydrophilic, organophobic layer on said sheet resulting from the reaction of the aluminum oxide coating and an alkali metal silicate applied to said coating, and a light-sensitive coating over said layer consisting essentially of a film-forming polyvinyl resin, and a light-sensitizing agent for insolubilizing said resin upon exposure of said light-sensitive coating to ultra-violet light, and having the properties of being hydrophobic and -organophilic after exposure to ultraviolet light.

3. A sensitized photographic printing plate comprising an aluminum sheet having a surface which has been treated to form an aluminum oxide coating on said surface; a IWater-insoluble, hydrophilic, organopholbic layer on said sheet resulting from the reaction of the aluminum oxide coating and an alkali metal silicate applied to said coating, and a light-sensitive coating over said layer comprising a hlm-forming polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and mixtures thereof, said resin containing a light-sensitizing agent which insolubilizes the resin under the action of ultra-violet light, said light-sensitive coating having the properties of being hydrophobic and organophilic after exposure to ultra-violet light.

4. A sensitized photographic printing plate comprising an aluminum sheet having a surface which has been anodized to for-m an aluminum oxide coating on said surface and which has been sealed, a Water-insoluble, hydrophilic, organophobic layer on said sheet resulting from the reaction of the aluminum oxide coating on said sheet and sodium silicate applied to said coating, and a Watersoluble, light-sensitive coating over said layer comprising polyvinyl alcohol containing a light-sensitizing agent of the class consisting of ammonium bichromate, sodium bicromate, potassium bichromate' and cupric bichromate, said light-sensitive coating having the properties of being water-insoluble, hydrophobic and organophilic after exposure to ultra-violet light.

5. A sensitized photographic printing plate as described in claim l, wherein said layer is an aluminosilicate structure in the nature of a zeolitic molecular sieve permitting small molecules, such as those of Water, nitrogen and ammonia to be adsorbed and rejecting larger molecules, such as those contained in printing ink.

6. A sensitized photographic printing plate comprising an aluminum sheet having a surface which has been treatatender.

ed to form an aluminum oxide coating on said surface, a 'water-insoluble, hydrophilic, organophobic layer on said sheet resulting from the reaction of the aluminum OXidecOatng and an alkali metal silicate applied to said coating, and a water-soluble, light-sensitive diazo resin coating over `said layer having the properties of being Water-insoluble, hydrophobic and organophilic upon exposure to ultra-Violet-light.

7. A photographic printing plate comprising an aluminum sheet which has been treated to form an aluminum oxide coating thereon, a Water-insoluble, hydrophilic, organophobic layer on said sheet resulting `from the reaction of the aluminum oxide coating on said sheet and an alkali metal silicate applied to said coating, and a water-insoluble, image-forming coating over said layer having the properties of being Water-insoluble, hydrophobic and organophilic, said layer being exposed in the non-image areas of the plate between the areas of the image-forming coating. v f Y Y 8. A photographic printing plate comprising an aluminum sheet having a surface which has been treated to form an aluminum oxide coating on said surface, a waterinsoluble, hydrophilic, organopholbic layer on said sheet resul-ting from the reaction of the aluminum oxide coat- ,ing and an alkali metal silicate applied to said coating,

and a Water-insoluble, image-forming coating over said layer consisting essentially of a polyvinyl resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and a mixture thereof, which has been insolubilized by the action of a light-sensitizing agent thereon mixed therewith upon exposure to ultra-violet light, said imageforming coating having the properties of being waterinsoluble, hydrophobic and organophilic, said layer being exposed inthe non-image areas of the plate between the area-s of the image-forming coating.

9. A photographic printing plate comprising an aluminum sheet having a surface which has been treated to form an aluminum oxide coa-ting -on said surface, a waterinsoluble, hydrophilic, organophobic layer on said sheet resulting from the reaction of the aluminum oxide coating and sodium silicate applied to said coating, and a water-insoluble image-forming coating V'over said layer having the properties of being water-insoluble, hydrophobic and organophilic, said image-forming coating being formed by applying a Water-soluble coating of polyvinyl alcohol and a light-sensitizing agent which insolubiiizes said alcohol in the presence or ultra-violet light, exposing said water-soluble coating through a stencil or negative to render the image-forming areas of the latter coating water-insoluble and removing the water-soluble non-image areas of the latter coating by a solvent which does not dissolve the image-forming areas of the waterinsoluble coating to expose the non-image areas of said layer between the areas of the image-forming coating.

10. A photographic printing plate as described in claim 9, wherein the light-sensitizing agent is of the class consisting of ammonium bichromate, sodium bichromate, potassium bichromate and cupric bichromate.

11. A photographic printing plate comprising an alumiform an aluminum oxide coating on said surface and which has been sealed, a water-insoluble, hydrophilic, organophobic layer on said sheet resulting from the reaction of thealuminum oxide coating and an alkali metal silicate applied to said coating, and a water-insoluble image-forming coating of diazo resin over said layer having the properties of being water-insoluble, hydrophobic and organophilic, said water-insoluble coating being formed by applying a water-soluble coating of diazo resin, exposing said water-soluble coating of diazo Vresin to ultra-violet light through a stencil or negative to render the image-forming areas of the latter coating wateririsoluble and removing the Vwater-soluble non-image areas of the latter coating by a solvent to expose the nonimage areas of said layer between the areas of the imageforming coating.

12. The process of making a sensitized photographic plate comprising the steps of applying to an aluminum sheet having a coating of aluminum oxide, a water-solution Iof an alkali metal silicate to cause the silicate to react with the aluminum oxide to form a water-insoluble, hydrophilic, organophobic layer onV said sheet, drying the layer, and applying over the dry layer a light-sensitive coating having one solubility in relation to a solvent in a state before exposure to light and another solubility in relation to said solvent in another state after exposure to light, said light-sensitive material being solu'blc in said solvent in one of said states and being insoluble in said solvent and in water, hydrophobic and organophilic in its other state.

13. The process as described in claim 12, wherein said solution of alkali metal silicate is applied at a temperature to seal the aluminum oxide coating.

14. The process as described in claim 12, wherein the light-sensitive coating consists essentially of a polyvinyl nlm-forming resin of the class consisting of polyvinyl alcohol, polyvinyl acetate and a Vmixture thereof, and a light-sensitizing agent for insolubilizing said light-sensitive coating upon exposure to ultra-violet light.

15. The process as described in claim 14, wherein the light-sensitizing agent is a bichromate of the class consisting of ammonium bichromate, sodium bichromate, potassium bichromate and cupric bichromate.

16. The process as described in claim 12, wherein the light-sensitive coating is a diazo resin.

References Cited by the Examiner UNITED STATES PATENTS 2,119,031 5/38 Wescott lOl-149.2 2,179,245 11/39 Wood lOl- 149.2 X 2,184,289 12/39 Dangelmajer 101--149.2 X 2,507,314 5/50 Mason 101-149.2 2,732,796 1/56 Van Dusen 101-1492 2,922,715 1/60 Gumbinner 101-149.2 X 3,030,210 4/62 Chebiniak 101-149.2 X

WILLIAM B. PENN, Primary Examiner. DAVID KLEN, Examiner. 

7. A PHOTOGRAPHIC PRINTING PLATE COMPRISING AN ALUMINUM SHEET WHICH HAS BEEN TREATED TO FORM AN ALUMINUM OXIDE COATING THEREOFN, A WATER-INSOLUBLE, HYDROPHILIC, ORGANOPHOBIC LAYER ON SAID SHEET RESULTING FROM THE REACTION OF THE ALUMINUM OXIDE COATING ON SAID SHEET AND AN ALKALI METAL SILICATE APPLIED TO SAID COATING, AND A WATER-INSOLUBLE, IMAGE-FORMING COATING OVER SAID LAYER HAVING THE PROPERTIES OF BEING WATER-INSOLUBLE, HYDROPHOBIC AND ORGANOPHILIC, SAID LAYER-BEING EXPOSED IN THE NON-IMAGE AREAS OF THE PLATE BETWEEN THE AREAS OF THE IMAGE-FORMING COATING. 